Generally, driving elements such as a clutch in automatic transmission of a vehicle are manufactured using stainless steel with excellent elongation to promote processability. More specifically, the spline member of an automatic transmission, where parts are frequently contacted, is manufactured by using stainless steel considering only its elongation. However, the friction generated by the frequent contacts made at the time of operating clutches is unable to prevent the part from being weathered and transformed. One of such parts in an automatic transmission, where parts are subjected to contact to each other, is in the kick-down drum which permits transmission shifts by restricting a reverse sun gear at the time of making second gear and fourth gear shifts (FIG. 6).
In fact, it is common that automobiles with a mileage of 70,000–80,000 miles have problems in performing transmission shifts due to the wear-out in the front end of the spline member. The main reason for the above phenomenon lies in that the material used to manufacture a kick-down drum is a heat-treated SPHD having a hardness of 190 Hv while its counter part, reverse sun gear, has a surface hardness of 650 Hv.
The following table 1 shows the composition of SPHD used as a material for manufacturing a kick-down drum. The SPHD contains a small amount of carbon and thus it does not have a desired strength or hardness.
TABLE 1CSiMnPSCrNiCuTiFeSPHD0.031—0.240.0100.010————Rem.
The SPHD in the above Table 1 comprises mostly ferrite with an extremely small amount of pearlite, wherein it has hardness of 180–190 Hv, yield strength of 244 MPa, tensile strength of 364 MPa, and elongation of 41%, thus resulting in abrasion and breakage of gear teeth in the spline member. Therefore, as an alternative to improve abrasion resistance of the spline member, it has been made with a material having excellent heat treating property via carburization/heat treatment and welded to the kick-down drum to be used. However, this requires an additional process such as finishing work of welding and heat treatment, which increases the production costs and adds complexity to the existing process. The kick-down drum consists of two parts, drum part (made of SPHD) and boss part (made of SCr420H1), and the composition of the boss part is shown below in table 2.
TABLE 2CSiMnPSCrNiCuTiFePresent Invention0.340.160.770.0120.0010.100.013——Rem.
The kick-down drum has been manufactured so that the boss part (spline member) only is hardened via carburization/heat treatment and then welded to the kick-down drum to be used. However, this increases production cost due to the presence of an additional step, increasing the surface hardness (8307–836 Hv) by carburization, which then leads to increased abrasion to its counter part, cracks in the spline member when there is an excessive load due to hardening, and even in some situations causes the breakage of the spline member.